Forensic BiologyRichard Li, with additions and edits by Ruth Ballard

Lecture 2: Sources of DNA Evidence

Outline

DNA Biological evidence containing DNA

Blood Semen Saliva Hair Bone Teeth Touch/trace

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DNA

Most cells contain DNA Digital information molecule Information carried in sequence

(A,T,C,G) Mutations change the sequence over

time▪ Different forms = alleles

In animals, two types▪ Nuclear▪ Mitochondrial▪ Forensic biologists use both 3

DNA in Forensic Biology

Nuclear DNA Inherited from both parents Two copies per cell Pristine (full profile):▪ Fresh materials (semen, saliva, blood, bone); rooted

hairs▪ E.g. Semen stain taken 12 hours after rape▪ PD in the quintillions

Compromised (partial profiles) or completely degraded (no profile): ▪ Aged or damaged materials; hair shafts▪ E.g. Saliva on a cigarette butt exposed to rain for 1 week▪ PD reduced based on level of degradation

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DNA in Forensic Biology

Mitochondrial DNA Found inside the cell’s mitochondria▪ Powerhouses that provide the cell’s energy

Thousands of copies/cell Inherited only from the mother Can be obtained from most biological materials

(fresh or old)▪ E.g. Skull found in a field; body dumped 2 years prior▪ Usually possible to get full profile

PD low (1 in a few hundred is typical)▪ All maternally-linked relatives carry the same mtDNA

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DNA in Forensic Biology

Nuclear DNA is preferred due to high PD

BUT… Mitochondrial DNA may be the only

option for some samples And it’s much better than nothing!

The fewer cells and/or the more compromised the evidence, the less likely you can get a nuclear DNA profile

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Biological Evidence containing DNA

Blood Often shed at violent crime scenes Is usually the victim’s blood▪ Look for victim’s blood on suspect’s skin,

shoes, or clothing Consists of erythrocytes (red blood

cells), leucocytes (white blood cells), and platelets

Nuclear DNA obtained from leucocytes; erythrocytes and platelets lack nuclei and DNA 7

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Nucleated leukocyte surrounded by enucleated erythrocytes

Biological Evidence containing DNA

Semen Very common in sexual assaults▪ Recovered from skin, vagina, oral cavity,

rectum of victims▪ Condoms = “perfect” evidence item

Rich source of DNA▪ Sperm are “little bags of DNA” (more on this

later…) Persists inside/on victims for up to 72

hours Persists on inanimate objects up to many

years▪ E.g. Semen stain on carpet

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Biological Evidence containing DNA

Saliva Common in sexual assaults▪ Recovered from skin, labia, peri-oral regions

on victim Other common sources:▪ Cigarette butts▪ Back of envelopes and postage stamps▪ Rims of beer cans, drinking glasses

Contains lots of bacteria▪ If evidence not dried and stored properly, will

quickly degrade10

Biological Evidence containing DNA

Hair Root Shaft

Medulla Cortex Cuticle

Grows at approximately 0.3mm per day Three phases:

Anagen phase- growing phase (nuclear DNA) Catagen phase- follicle becomes inactive (nuclear

DNA) Telogen phase- growth cycle ends (mtDNA only)

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A hair follicle

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Cross section of a hair shaft

Biological Evidence containing DNA

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Telogen hairs lack roots People shed more than 200 telogen hairs

per day Look inside caps or on clothing or

furnitureHairs with rooted must be plucked or

pulled out by force Look in the hands of a victim after a

struggleProcedure:

Microscopy to identify phase Choose appropriate DNA testing method

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Hair morphology: Differences in human hair

color

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Hair morphology: Absence (above) or presence (below) or medulla

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Hair morphology: Distinct cuticle layering patterns

Biological Evidence containing DNA

Bone Spongy▪ Rich source of cells and DNA but…▪ Degrades quickly after death▪ Good source of nuclear DNA

Compact▪ Fewer cells than spongy bone but…▪ Degrades less quickly after death▪ Good source of mitochondrial DNA

Mass fatalities (e.g. 911) Sanding prior to DNA analysis to prevent

comingling of remains18

Biological Evidence containing DNA

Bone Rate of degradation of human remains

varies greatly with environmental conditions▪ Soft tissue is lost▪ Bones are more stable

Mass Fatalities-identify victims

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Biological Evidence containing DNA

Teeth Dentin- no cells Pulp Cavity▪ Contains lots of cells and

DNA if tooth is fresh Root▪ Contains root canal with

cells and DNA if tooth is fresh

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Biological Evidence containing DNA

In addition to DNA analysis, teeth can be used to:

Identify a person▪ Characteristics of teeth▪ Alignment▪ Overall structure▪ X-rays

Compare suspect’s with bite marks on victim▪ What else might be present on a bite mark

that could help identify a criminal? 21

Biological Evidence containing DNA

Touch/Trace DNA First described into 1997: “DNA Fingerprints

from Fingerprints” (Nature article) Sources:▪ Surface skin cells▪ Cells transferred onto hands from eyes and mouth

Low levels; usually partially degraded Often contains DNA from more than one person▪ DNA mixtures▪ Difficult to interpret (is it really related to the crime?)▪ E.g. Cell phones, keyboards, doorknobs, clothing

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SemenSalivaBloodHairBoneTrace/touch

Primary Biological Evidence in Cases Reviewed by Dr. Ballard in 2012